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Published 1994 | Published
Journal Article Open

Time-resolved Microwave Conductivity. Part 1.—TiO_2 Photoreactivity and Size Quantization


Charge-carrier recombination dynamics after laser excitation are investigated by time-resolved microwave conductivity (TRMC) measurements of quantum-sized (Q-) TiO_2, Fe^(III)-doped Q-TiO_2, ZnO and CdS, and several commercial bulk-sized TiO2 samples. After pulsed laser excitation of charge carriers, holes that escape recombination react with sorbed trans-decalin within ns while the measured conductivity signal is due to conduction-band electrons remaining in the semiconductor lattice. The charge-carrier recombination lifetime and the interfacial electron-transfer rate constant that are derived from the TRMC measurements correlate with the CW photo-oxidation quantum efficiency obtained for aqueous chloroform in the presence of TiO_2. The quantum efficiencies are 0. 4 % for Q-TiO_2, 1. 6 % for Degussa P25, and 2. 0 % for Fe^(III)-doped Q-TiO_2. The lower quantum efficiencies for Q-TiO_2 are consistent with the relative interfacial electron-transfer rates observed by TRMC for Q-TiO_2 and Degussa P25. The increased quantum efficiencies of Fe^(III)-doped Q-TiO_2 and the observed TRMC decays are consistent with a mechanism involving fast trapping of valence-band holes as Fe^(IV) and inhibition of charge-order recombination.

Additional Information

© 1994 Royal Society of Chemistry. Paper 4/02296C; Received 18th April, 1994. We are indebted to Prof. Nathan S. Lewis for the loan of microwave components, to Prof. Geoffrey A. Blake for the use of the excimer laser and to Dr. Detlef W. Bahnemann for providing the Sachtleben Chemie samples. We are grateful to ARPA and ONR {NAV 5 HFMN N0001492J1901) for financial support. S.M. is supported by a National Defense Science and Engineering Graduate Fellowship. H.H. wishes to thank NATO/DAAD for financing a research visit at the California Institute of Technology. Nicole Peill, Dr. Amy Hoffman and Dr. Andreas Termin provided valuable support and stimulating discussion.

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August 20, 2023
August 20, 2023